1. Field of the Invention
The invention relates to an apparatus for testing an ATM type telecommunications network and for measuring the performance of an ATM connection. It may be recalled that an ATM (Asynchronous Transfer Mode) telecommunications network enables the transmission of digital data elements having varied bit rates that could be high. These bit rates could indeed go up to 155 Mbits per second and even more.
2. Description of the Prior Art
Transmission in ATM mode is done by the transfer of information blocks of a constant length. The elementary quantity of useful information transmitted is 48 bytes. The information block transmitted in ATM is called a cell and identified by a five-byte label called a header. The total length of a cell is therefore 53 bytes.
The technique of transmission in ATM mode is based on the placing of the information in cells. The transfer network then takes charge of switching, multiplexing and transmitting these cells.
The transfer network is a shuffling or switching network that uses the ATM transmission mode and does not directly manage the basic communication, placed in the form of cells, but a group of multiplexed communications in a bundle of cells.
Apart from the transfer function, the network also takes charge of generating and terminating these bundles in service or network multiplexers.
Finally, the network makes use of the independence of the asynchronous transfer mode (ATM layer) with the transmission systems to use the plesiochronous and the synchronous network (physical layer).
The parameters of quality of an ATM connection have been defined in the CCITT Recommendation No. I356.
On the basis of this definition of the parameters, the present Applicant has defined two types of tools needed for the testing of network elements and for the measurement of the performance characteristics of the network:
a tool for testing the ATM functions whose role is to measure the performance characteristics of a given connection by simulating load conditions on the other connections. To this end, there is provision for testing mechanisms for the different layers of the protocol, namely ATM and AAL (ATM adaptation layer). There is also provision for simulating the influence of the physical layer by the controlled generation of configuration errors etc., and for simulating the influence of the ATM layer by generating jitter, insertion or cell loss phenomena.
The second tool is a tool for the testing of operational functions enabling the validation of the capacities of equipment and of the network, to fulfil the specified functions: interface, maintenance and signalling.
To date, there is no apparatus that can be used to measure the performance parameters of an ATM network and test the operational functions of the network.
The problem that the Applicant has sought to resolve relate to a new need consisting in the devising of an ATM testing and measuring apparatus for the putting into operation of an ATM connection whether at the level of the testing of the network element or that of the measurement of the performance of the network. According to one aspect of the invention, the measuring apparatus consists of either one or two items of equipment. The measuring apparatus has one piece of equipment when the network tests are performed in a loop, as shown in FIG. 1Aa. It has two pieces of equipment when tests are performed from point to point between two distant centers as shown in FIG. 1Ab. Only one piece of equipment is used for monitoring functions in FIG. 1Ac. To facilitate the description hereinafter, it shall be assumed that the measuring apparatus has two pieces of equipment, one transmitter and one receiver-analyzer hereinafter called a receiver.